Defining the mechanisms by which stress in the environment during pregnancy promotes changes in development is critical in identifying factors predictive of disease risk or resilience. One major consistency across prenatal insults is the increased vulnerability of males. In this proposal, we utilize our mouse model of early prenatal stress (EPS) to examine sex-specific placental transcriptional regulation. In our EPS model, male, but not female, offspring present with increased stress sensitivity, including increased HPA stress axis activity, reduced post-weaning growth, and hypothalamic mitochondrial dysfunction. Sex differences in the placental function are likely to produce sex-specific transplacental signals to the developing fetal brain. Sex differences in the placenta begin with sex chromosomes. Through a genome-wide screen following maternal stress, we identified the X-linked gene, OGT, as causal in programming the male-specific stress phenotype via its regulation of the histone transcriptional repressive mark, H3K27me3. This proposal uses innovative approaches to determine the mechanisms by which the female placenta is able to restrict transcriptional responses to stress in the environment, where males are not, thus placing the male developing brain at greater risk prenatally. The transplacental signals received by the developing brain appear to be related to energy availability and impact metabolic and mitochondrial programming. Of key translational importance, we have also found the same biochemical and molecular outcomes are predicted by fetal sex in human placental tissue. Therefore, our proposal will focus on defining the causal importance of H3K27me3 in risk for developmental changes in response to stress, identify the sex-specific transplacental signals resulting from these changes in placental function using ex vivo perfusion, and determine the cellular compartment and mechanism by which these changes promote hypothalamic mitochondrial reprogramming and the EPS phenotype.